1. Field of the Invention
The present invention relates to a semiconductor device and, more specifically, to a semiconductor device with a circuit for detecting an abnormal waveform of an input signal and preventing the signal from being transmitted.
2. Description of the Related Art
Conventionally, a semiconductor device, especially a semiconductor memory has been often supplied with a signal having an abnormal waveform whose pulse width is shorter than a fixed period due to noise on a mounting substrate, the skew of an address signal output from a CPU, etc. In general, the semiconductor memory is often supplied with a signal whose pulse width is 5 ns or less.
If a semiconductor memory receives a signal having an abnormal waveform, it responds to the abnormal waveform to cause the system to malfunction. In order to resolve this problem, the following method has been adopted. A noise filter is provided on an input signal path to prevent a semiconductor memory from responding to an abnormal waveform (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-295988).
The above method however has the problem. If a semiconductor memory is prevented from responding to an input signal whose pulse width is 5 ns or less, its response is delayed by 5 ns even when the memory is supplied with a normal waveform. In a system that requires a high-speed operation whose access time ranges from 30 ns to 50 ns, the operating margin becomes very narrow if the access time is 5 ns delay. Accordingly, a method with no penalty for access time is required.